Long, wide belts that circulate in the paper machine and support the paper webs are used in paper machines. These belts are usually textile felts or cloths that comprise as their basis a yarn structure, for example a woven or knitted fabric. Instead of a yarn structure of this kind, it is also possible to provide yarn layers having at least one ply of longitudinal yarns and at least one ply of transverse yarns intersecting the longitudinal yarns, the longitudinal and transverse yarns being interconnected at the intersection points by means of adhesive material, positive engagement, and/or fusing (EP 1 357 223 A1; EP 1 359 252 A1; EP 1 359 251 A1).
For use in the sheet-forming region and the dryer section of a paper machine, the paper machine belt comprises the yarn structure as such. For use in the press section of a paper machine, one or more fiber layers are needle-felted onto the yarn structure.
In many cases the paper machine belts are not endless, but rather are manufactured in a specific length. The front edges at the ends are then, prior to insertion into the paper machine or even in the paper machine itself, interconnected via a seam. The so-called pintle wire seam has proven particularly successful in this context. With this seam, coupling elements having a plurality of coupling eyelets are provided on the mutually facing front edges of the belt, and for closure are made to overlap with one another in such a way that they engage in comb fashion into one another and a continuous passage extending in the transverse belt direction is formed. A pintle wire is then slid into this continuous passage and connects the two front edges in the manner of a hinge.
A variety of systems are known for embodying the coupling eyelets. For example, the coupling eyelets can be formed by looping back the end regions of the paper machine belt (U.S. Pat. No. 2,077,891; U.S. Pat. No. 3,309,790). Individual yarns can also be looped back and woven back in (U.S. Pat. No. 5,148,838).
As an alternative to this, the coupling eyelets can be formed by U-shaped clamps that hook into the ends of the paper machine belt. The seams produced with the aid of such coupling eyelets are referred to as clipper seams (DE-A-2 256 244; U.S. Pat. No. 4,344,209). It is additionally known to form each of the coupling eyelets by way of a U-shaped sheet-metal part, the limbs of each sheet-metal part being connected to the associated front edge of the belt (U.S. Pat. No. 3,972,105). A plurality of cutouts are shaped into the sheet-metal parts, so that coupling projections having coupling eyelets are created.
Another alternative is to form coupling eyelets by way of coupling coils respectively secured onto the front edges (DE-A-2 256 244, U.S. Pat. No. 4,477,435; EP 0 158 907 B1; EP 0 564 436 A1). Connection of the coils to the end faces of the paper machine belt occurs either by means of special seam yarns or by way of the longitudinal yarns of the yarn structure, by the fact that the latter loop around the coils and are woven back. Yarns or wires around which the longitudinal threads are laid can be placed into the coils (DE-A-2 256 244, FIG. 8e; EP 0 185 907 B1, FIG. 10).
Also known as coupling elements forming coupling eyelets, instead of coupling coils, are special shaped parts made of plastic that are respectively connected to the longitudinal yarns of the paper machine belt (WO 96/34146; DE-A-199 44 864; GB-A-2 231 838). The longitudinal yarns are looped back and, as a rule, woven back in for that purpose. Transverse wires, which reinforce the shaped parts or around which the longitudinal yarns are laid, can be placed into the shaped parts (DE-A-199 44 864, FIGS. 1 and 6).
For paper machine belts of the species having a yarn layer, complementary coupling elements to which coupling members interconnected in hinge fashion are attached have been proposed for connecting the front edges, the connection to the front edges being created via insertion connections with the longitudinal yarns (EP-A-1 357 224). The coupling elements can be embodied as coupling strips that extend over the entire width of the paper machine belt or only a portion of that width, several coupling strips then being provided next to one another. It has been found however, that this type of connection is not strong enough in cases in which the paper machine belt is exposed to large longitudinal forces.